1. Field of the Invention
This invention relates to an image processing technique for recovering reading faults in digital image data read from an image recording medium by a scanner or the like, such faults being due to imperfections present on the image recording medium such as scratches, dust, stains and the like.
2. Description of the Related Art
Printers for capturing images recorded on photographic film serving as an image recording medium and outputting the images on printing paper include analog printers and digital printers. An analog printer exposes printing paper by using light transmitted through images on photographic film. A digital printer acquires, by means of a scanner or the like, digital image data (hereinafter called “image data”) from light transmitted through images on photographic film, and exposes printing paper based on this image data. The image data here consists of a plurality of pixels arranged in a matrix. Each pixel has a pixel value of each basic color (e.g. R, G and B), and also a pixel value of infrared light where infrared light also is used.
With both analog and digital printers, when the photographic film has scratches, dust or stains (hereinafter collectively called “imperfections”), the images printed on the printing paper have faults such as density variations or deficiencies. Conventionally, therefore, in order to ease such faults, both analog and digital types use diffused light for transmission through photographic film. Furthermore, the digital printer can eliminate influences of the above imperfections from the image data by putting the image data to image processing. For this aspect, reference may be made to the following patent documents: Japanese Unexamined Patent Publication H6-28468 (corresponding U.S. Pat. No. 5,266,805), Japanese Unexamined Patent Publication 2000-341473, Japanese Unexamined Patent Publication 2000-349968, and Japanese Unexamined Patent Publication 2001-157003 (corresponding U.S. publication US2003127597A).
The image processing for eliminating influences of the above imperfections, i.e. image processing for recovering faults due to the above imperfections in digital image data read, will be described. First, infrared light is transmitted through photographic film. The infrared light is scattered by the above imperfections on the photographic film, but is not easily influenced by an image recorded on the photographic film. Therefore, infrared image data obtained by a scanner or the like that reads the infrared light transmitted through the photographic film includes mainly information about the imperfections. In the input image, pixels influenced by the above imperfections tend to have low pixel values of infrared light (luminance signal values based on the infrared light; hereinafter infrared pixel values). Thus, the pixels with relatively low infrared pixel values may be determined to be pixels influenced by the imperfections (hereinafter “defective pixels”).
Next, visible light is transmitted through the photographic film. Visible light is influenced by the image on the photographic film, and is scattered by the above imperfections on the photographic film. Thus, regarding the defective pixels, the pixel values (R pixel value, G pixel value and B pixel value) for the respective colors of R (Red), G (Green) and B (Blue) acquired by a scanner or the like reading the light transmitted through the photographic film are lowered to a level similar to the above infrared pixel values.
It is therefore possible to recover faults in the image read due to the above imperfections by adding amounts of reduction of the infrared pixel values as amounts of correction to the R, G and B pixel values regarding the above defective pixels.
However, an image processing using the infrared image data (formed of infrared pixels having the infrared pixel values) is effective for defective pixels influenced by scratches present on the base surface of photographic film, but has difficulties in removing the influence on pixels exerted by scratches present on the emulsion surface (hereinafter “emulsion scratches”). The reason is as follows.
Scratches on the emulsion surface of photographic film refer to a state of an image being damaged by scraping off of coloring matters forming the photographic film. The emulsion surface of film refers to sensitive layers with a red sensitive layer (cyan coloring matter), a green sensitive layer (magenta coloring matter) and a blue sensitive layer (yellow coloring matter) laid successively on the base surface as shown in FIG. 8.
When a scratch is formed on the emulsion surface, as shown in FIG. 8, the layers are scraped off successively from the blue sensitive layer onward, and in varied amounts. Thus, the R, G and B pixel values of pixels influenced by the emulsion scratch (hereinafter called “emulsion scratch pixels”) increase by different amounts for the respective color components, compared with a state of being free from emulsion scratches. It is therefore difficult to remove the influence of the emulsion scratch from the emulsion scratch pixels for the R, G and B pixel values by a process of adding the amount of reduction of the infrared pixel value to each.
Thus, the above emulsion scratch pixels require a process different from the process using the infrared pixel values. According to the description in paragraph [0104] of Japanese Unexamined Patent Publication 2001-78038, it is considered preferable to remove the influence of an emulsion scratch by an interpolating process for the pixel values of the above emulsion scratch pixels.
The above interpolating process is intended to remove the influence of the above imperfections by producing corrected R, G and B pixel values of each defective pixel from the R, G and B pixel values of normal pixels (i.e. pixels not influenced by the above imperfections) lying around the defective pixel such as the emulsion scratch pixel. However, although the above interpolating process can reproduce color and density free from defects, an output image will be smoothed with low granularity. That is, the part of the output image corresponding to the above defective pixels has less graininess (noise component) than other parts, so that the part corresponding to the above defective pixels will become conspicuous.
Thus, when an interpolating process is carried out for the above defective pixels, the pixels subjected to the process will stand out in the output image. The output image can hardly be said a corrected image free from the influence of the above imperfections.